Tube Bias issue.

Marty (Viridian), you may find it helpful to take a look at the biasing procedure described in the manual for this amp. I would think that the wildly fluctuating meter reading is most likely a true indication that the bias is wildly fluctuating.

Rluthy, it's hard for me to envision how a speaker defect could be causing this problem. Bias current is DC, which flows on the primary side of the output transformers, while the speakers are of course connected to the secondary side. And transformers don't pass DC, aside possibly from extremely miniscule amounts of leakage. And in any event it sounds like the speakers don't have any defects, based on their performance when the amp is in triode mode.

My speculative guess is that some sort of oscillation is occurring, involving an interaction between the amp and either the speakers or the preamp. An oscillation involves a path that is a closed loop, and I'm guessing that part of that loop may include the effects of the AC current draw of either the Zeus amp or the speaker's bass amp on the AC line voltage being seen by either the other of those two components or the preamp. Some thoughts and questions:

1)Do the woofers appear to be moving when the problem occurs?

2)Are you following the manual's instruction to turn off the preamp when doing the adjustments?

3)Even though the manual says it's ok to change from triode mode to UL mode while the amp is powered up, have you tried changing into UL mode while the amp is turned off, and then turning the amp on and observing the bias meters?

4)Try plugging the speakers into a different AC branch than the one the amp and the rest of the system are plugged into, using a long extension cord if necessary.

5)Beyond that, all that occurs to me to suggest at this point is to go to Radio Shack and pick up either a couple of cheap speaker drivers, or alternatively a pair of 8 ohm resistors rated to handle substantial power levels (I think they sell one rated for 20 watts), and seeing what happens with those connected in place of the Summits. Although if the bias readings do not fluctuate with those dummy loads in place, per my comments above it would not necessarily be indicative of a speaker defect. My suspicion in that case would be that some kind of interaction between the amp and the speakers is resulting in an oscillation.

Good luck. Regards,
-- Al

Ralph, note in an earlier post that the OP indicated he tried both of the taps the amp provides (4 and 8 ohms). Also, he indicated that the problem occurred even if the AC power plugs of the speakers were disconnected. That would seem to rule out the powered woofer section of the speaker as having anything to do with the problem. And I'm not sure that an unpowered electrostatic element can be microphonic.

12-15-14: Rluthy57
I disconnected the Martin Logans and hooked up a small pair of 6 ohm Phillips 2 way speakers into the amps 8 ohm taps and the ultra-linear bias fluctuation problem went away. Wouldn't this tell me that the amp is ok and the problem is with the Martin Logan Summit speakers? If it's the speakers I still don't understand why they sound so good when the amp is played in triode mode.

My speculation, as stated in my initial post, was:

... if the bias readings do not fluctuate with those dummy loads in place, per my comments above it would not necessarily be indicative of a speaker defect. My suspicion in that case would be that some kind of interaction between the amp and the speakers is resulting in an oscillation.

If an oscillation is occurring, at some inaudible frequency, the amp's feedback loop would presumably be involved, and therefore pretty much the entire signal path of the amp would presumably be involved. I see that the small signal tubes used in the amp are two 12AX7's and four 12AU7's. What type and make of these tubes are you using, and are they the tubes that were originally supplied with the amp? Just a guess, but perhaps changing the 12AX7's to lower gain equivalents might resolve the problem.

Regards,
-- Al

Awesome! Glad to have helped solve this extremely frustrating problem. And thanks for the nice words :-)

Ralph, in the interest of optimizing sonics do you think it would be worthwhile for Robert to try a pair of 5751's in the 12AX7 locations? As you no doubt are aware, 5751's are commonly used as lower gain substitutes for 12AX7's, having a "u" (amplification factor) of about 70% of that of the 12AX7. That is much higher than the u of the 12AU7, of course, so there is no guarantee that the problem wouldn't return with them. But other parameters such as Rp and Gm are much closer to those of the 12AX7 than the corresponding parameters of the 12AU7, so I'm thinking that they may therefore be more optimal sonically.

Looking at an older GE tube manual I have, though, I note that the 5751 has a rated maximum plate dissipation of 0.7 watts, while the corresponding figures for the 12AX7 and 12AU7 are 1.2 and 2.75 watts respectively (for both the "A" and no suffix versions). I suspect that is unlikely to be an issue, but having no knowledge of the specific design I'm not totally certain.

Best regards,
-- Al